In recent years, due to the fact that a digital versatile disc (DVD) can record digital information with a recording density about 6 times higher than a compact disc (CD), the DVD is drawing attention as an optical recording medium having a large capacity. However, along with the development of information to have a larger capacity, a high-density optical recording medium has been desired. Here, in order to achieve higher density than the DVD (wavelength 660 nm, numerical aperture (NA) 0.6), it is necessary to shorten the wavelength of a light source and to enlarge the NA of an objective lens. For example, when using a blue laser of 405 nm and an objective lens of NA 0.85, a recording density 5 times higher than that of the DVD can be achieved.
However, a high-density optical disc device using the above-mentioned blue laser has an extremely strict reproduction margin, so that quantum noise of the light source becomes a problem. Therefore, JP2000-195086A proposes an optical head that can suppress the quantum noise of a semiconductor laser to a low level and perform excellent reproduction with low noise while suppressing the power on the surface of an optical disc to a low level, thus preventing the optical disc from deteriorating or the data from being deleted and so forth.
Here, one example of the above-mentioned conventional optical head will be described with reference to the drawing.
FIG. 20 is a schematic view showing the configuration of a conventional optical head disclosed in JP2000-195086A.
In this drawing, 161 is a light source, 162 is an intensity filter, 163 is a beam splitter, 164 is a collimator lens, 165 is a mirror, 166 is an objective lens, 167 is an optical disc, 168 is a multilens, and 169 is a photodiode.
The light source 161 is a GaN-type blue luminous semiconductor laser, which is a light source emitting a coherent light for recording and reproduction to a recording layer of the optical disc 167. The intensity filter 162 is an element on which an absorption film is formed and is arranged to be inserted into and taken out from an optical path. The beam splitter 163 is an optical element for splitting light; the collimator lens 164 is a lens that converts a divergent light emitted from the light source 161 into a parallel light; the mirror 165 is an optical element for reflecting an incident light and directing it in the direction of the optical disc 167; the objective lens 166 is a lens that focuses light on the recording layer of the optical disc 167; the multilens 168 is a lens that focuses light on the photodiode 169; and the photodiode 169 receives light reflected by the recording layer of the optical disc and converts the light into an electric signal.
The operation of the optical head having the aforementioned structure will be explained. Here, the intensity filter 162 is inserted into the optical path at the time of reproduction and taken out from the optical path at the time of recording. With respect to the light emitted from the light source 161, the light quantity is attenuated by passing through the intensity filter 162 at the time of reproduction, while the light quantity is not attenuated at the time of recording since the intensity filter 162 is taken out from the optical path. Next, the light transmitted through the intensity filter 162 (the light emitted from the light source at the time of recording) is reflected by the beam splitter 163 and converted into a parallel light by the collimator lens 164. The light converted into the parallel light is reflected by the mirror 165 and focused on the optical disc 167 by the objective lens 166. Next, the light reflected from the optical disc 167 is transmitted through the objective lens 166, reflected by the mirror 165, transmitted through the collimator lens 164, transmitted through the beam splitter 163 and focused on the photodiode 169 by the multilens 168. Using the astigmatism method, the photodiode 169 outputs a focus error signal showing the focused state of the light on the optical disc 167 or a tracking error signal showing the irradiated position of the light.
Based on the focus error signal, focus control means not shown in the drawing controls the position of the objective lens 166 in its optical axial direction so that the light is collected constantly on the optical disc 167 in the focused state.
Furthermore, based on the tracking error signal, tracking control means not shown in the drawing controls the radial position of the objective lens 166 so that the light is focused on a desired track on the optical disc 167.
In addition, the photodetector 169 reproduces information recorded in the optical disc 167.
According to this configuration, it is possible to perform reproduction by suppressing the power on the surface to a low level that is incapable of causing deterioration of the optical disc or deletion of the data, while setting the power of the light source to a power with sufficiently low quantum noise, and to perform recording by using the power of the light source as it is.
However, in the optical head having the aforementioned structure, the intensity filter 162 needs to be inserted and taken out at the time of switching between recording and reproduction, and in the case where recording needs to be performed instantaneously after an address reproduction, the speed of inserting and taking out the intensity filter 162 becomes a problem. For example, the next generation high-density optical disc with a higher density than the DVD requires a switching time of about 100 n seconds, but such a speed is extremely difficult to be achieved by inserting and taking out in a mechanical manner.
Furthermore, in order to insert and take out the intensity filter 162, a system (mechanism) for implementing this operation is necessary, and thus, the optical head tends to become larger in size, so that this configuration is not suitable for miniaturization of the optical head.